Re: Request for backport fd6bc19d7676 to 4.14 and 4.19 branch

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Aug 17, 2021 at 06:47:45PM +0000, David Chen wrote:
> 
> 
> > -----Original Message-----
> > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Sent: Monday, August 16, 2021 11:16 PM
> > To: David Chen <david.chen@nutanix.com>
> > Cc: stable@vger.kernel.org; Paul E. McKenney <paulmck@linux.vnet.ibm.com>; neeraju@codeaurora.org
> > Subject: Re: Request for backport fd6bc19d7676 to 4.14 and 4.19 branch
> > 
> > On Mon, Aug 16, 2021 at 10:02:28PM +0000, David Chen wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Sent: Monday, August 16, 2021 12:31 PM
> > > > To: David Chen <david.chen@nutanix.com>
> > > > Cc: stable@vger.kernel.org; Paul E. McKenney
> > > > <paulmck@linux.vnet.ibm.com>; neeraju@codeaurora.org
> > > > Subject: Re: Request for backport fd6bc19d7676 to 4.14 and 4.19 branch
> > > >
> > > > On Mon, Aug 16, 2021 at 07:19:34PM +0000, David Chen wrote:
> > > > > Hi Greg,
> > > > >
> > > > > We recently hit a hung task timeout issue in synchronize_rcu_expedited on
> > > > 4.14 branch.
> > > > > The issue seems to be identical to the one described in `fd6bc19d7676
> > > > > rcu: Fix missed wakeup of exp_wq waiters` Can we backport it to 4.14 and
> > > > 4.19 branch?
> > > > > The patch doesn't apply cleanly, but it should be trivial to resolve,
> > > > > just do this
> > > > >
> > > > > -		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp-
> > > > >expedited_sequence) & 0x3]);
> > > > > +		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
> > > > >
> > > > > I don't know if we should do it for 4.9, because the handling of sequence
> > > > number is a bit different.
> > > >
> > > > Please provide a working backport, me hand-editing patches does not scale,
> > > > and this way you get the proper credit for backporting it (after testing it).
> > >
> > > Sure, appended at the end.
> > >
> > > >
> > > > You have tested, this, right?
> > >
> > > I don't have a good repro for the original issue, so I only ran rcutorture and
> > > some basic work load test to see if anything obvious went wrong.
> > 
> > Ideally you would be able to also hit this without the patch on the
> > older kernels, is this the case?
> > 
> So far we've only seen this once. I was able to figure out the issue from the vmcore,
> but I haven't been able to reproduce this. I think the nature of the bug makes it
> very difficult to hit. It requires a race with synchronize_rcu_expedited but once
> the thread hangs, you can't call it again, because it might rescue the hung thread.

I would like a bit more verification that this is really needed, and
some acks from the developers/maintainers involved, before accepting
this change.

thanks,

greg k-h